This invention relates to a synchronizing signal reproducing circuit in a disc playback device for taking out a synchronizing signal from a signal reproduced from a disc, such synchronizing signal being used for a reference signal in various controls such as a disc motor control in disc playback devices for playing back discs such as an optical type video disc and a Compact Disc in the Compact Disc Digital Audio System.
In a disc playback device for a video disc or a Compact Disc, a synchronizing signal (i.e., horizontal synchronizing signal) is taken out of a reproduced signal from the disc and is used, for example, for rotation control of a disc motor by PLL by comparing this synchronizing signal in phase with a reference clock signal. For taking a synchronizing signal out of a reproduced signal from a video disc, for example, a composite video signal is reproduced from the video disc, a synchronizing signal component is separated from this composite signal by comparing it at a predetermined threshold value and an assumed synchronizing signal is obtained by edge-detecting this component. Since there is a possibility that this assumed synchronizing signal contains an equalizing pulse and noise, a so-called "window" is established, which is a predetermined range on the time axis in which the synchronizing signal is expected to occur, and a signal which has occurred in this window only is taken out as a true synchronizing signal. If there is no signal occurring in this window, a substitution signal is provided to serve as a synchronizing signal.
In prior art devices, the window is established in such a manner that, as shown in FIG. 3a, a future side range and a past side range of the window with respect to the timing at which a synchronizing signal is expected to occur are of the same time length, i.e., the expected synchronizing signal occurs at a central point in the window.
In the system using this window, a first signal occurring in the window is deemed to be effective and a next window is established on the basis of this first signal. Accordingly, when a noise or an equalizing pulse occurs before a true synchronizing signal in the window as shown in FIG. 3b, a next window is established on the basis of this noise or equalizing pulse so that the position of the window is shifted to the past side as shown in FIG. 3c. Besides, it is judged in this case that the disc motor is rotating at a greater speed than a predetermined one because the signal has been obtained at an earlier timing than a timing at which the synchronizing signal is expected to occur so that a control is performed to reduce the speed with a result that occurrence of a next synchronizing signal is further delayed. Thus, the window and the synchronizing signal are shifted in directions in which they are spaced away from each other with a resulting tendency that a next synchronizing signal does not occur in the window. If this state is brought about, the synchronizing signal can no longer be detected. If the window range is unduly enlarged for preventing occurrence of this state, it will result in the disadvantage that it becomes easier to pick up noises in this enlarged window.
It is, therefore, a first object of the invention to provide a synchronizing signal reproducing circuit in a disc playback device which, when a noise has occurred before a true synchronizing signal in the window, is capable of including a subsequent synchronizing signal within the window without unduly enlarging the window range.
In the system using this window, there sometimes occurs a state in which absence of the synchronizing signal continues for many consecutive periods. If this state occurs, it is often more proper to judge that the reproduced signal is not in synchronization (i.e., in the state of "lockout") than to consider that some synchronizing signals are missing due to dropout or the like.
For coping with this problem, prior art devices remove the window immediately upon occurrence of such a state, treat a signal which occurs first after the removal of the window as a synchronizing signal unconditionally and establish the window anew on the basis of this signal thereby to bring the reproduced signal into a synchronized state again.
The prior art devices however have the disadvantage that, if the window is removed immediately upon making the judgement that the lockout state exists, there arises the possibility that the device treats a false signal such as an equalizing pulse or noise as a synchronizing signal with a resulting erroneous operation of the device.
It is, therefore, a second object of the invention to provide a synchronizing signal reproducing circuit in a disc playback device which, when a lockout state exists, is capable of bringing the reproduced signal into a synchronized state again without causing an erroneous operation due to an equalizing pulse or a noise.